Apparatuses and methods are used when visualizing and inspecting printed webs or printed sheets. After printing, it is necessary to check the printed result for the purposes of quality assurance. According to a first possibility, the sheets or webs are moved through under video cameras, in which case the video cameras record an image and an operator can check this. In addition, use is also made of automatic image checking systems in which a computer carries out an image comparison.
In particular applications, for example, in so-called finishing machines for the label industry, the demands imposed on print inspection are generally not particularly high and so discrete optical sensors for detecting missing labels or gross errors are often used here. This non-imaging automatic rough inspection is normally assisted by a manual inspection with the aid of a stroboscope. The quality of this inspection method substantially depends on the capabilities of the machine operator. In addition, the use of stroboscopes harbors health risks such as epilepsy, eye damage, etc.
In video systems, use is nowadays made of traversing video cameras which approach the desired position within the print repeat using positioning drives or else manually and record images there in a flash-controlled manner. These images are then transferred to a superordinate computer where they are processed further—for example, the print can be evaluated or inspected in terms of color, position and content—and are then displayed on a screen.
During operation, the operator is able to display the approached position in an enlarged manner. He can either obtain an extensive impression of the print quality, in particular with respect to the color, using a zoom factor of 1.0 or can check the quality of the printing raster and the register accuracy using high magnification, for example, with a zoom factor of 10 to 16. The zoom function can be achieved in two ways: either the use of a motorized zoom lens or the combination of two imaging sensors in order to achieve electronic zoom using CCD or CMOS cameras. The latter principle is disclosed in U.S. Patent Publication No. 2006/0239510 A1 and EP Patent No. 2003443 A1.
Traversing video cameras with mechanical or electronic zoom record only a relatively small area of the printed material flow. This is in practice only a few percent, which is why the systems are rather poorly suited to inspecting the print. Use is additionally made of matrix cameras which, however, presuppose extensive homogeneous illumination. This cannot be achieved in practice, in particular for highly reflective materials or holograms. Therefore, errors in the printed image cannot be reliably detected, whether it is not possible to clearly determine whether an abnormality can be attributed to an error or reflection.
There are also special line scan cameras, but these are expensive. The advantage of line scan cameras is that highly reflective materials or holograms can also be recorded when recording a single line.